[runtimes] Don't try passing --target flags to GCC

[ldionne]

Summary

When a target triple is specified in CMake via XXX_TARGET_TRIPLE, we tried
passing the --target=<...> flag to the compiler. However, not all compilers
support that flag (e.g. GCC, which is not a cross-compiler). As a result,
setting e.g. LIBCXX_TARGET_TRIPLE=<host-triple> would end up trying to
pass --target=<host-triple> to GCC, which breaks everything because the
flag isn't even supported.

This commit only adds --target=<...> & friends to the flags if it is
supported by the compiler.

One could argue that it's confusing to pass LIBCXX_TARGET_TRIPLE=<...>
and have it be ignored. That's correct, and one possibility would be
to assert that the requested triple is the same as the host triple when
we know the compiler is unable to cross-compile. However, note that this
is a pre-existing issue (setting the TARGET_TRIPLE variable never had an
influence on the flags passed to the compiler), and also fixing that is
starting to look like reimplementing a lot of CMake logic that is already
handled with CMAKE_CXX_COMPILER_TARGET.

[phosek]

LGTM

[ldionne]

Will ship once CI passes.

libcxx/CMakeLists.txt

 
 # Target flags ================================================================
 # These flags get added to CMAKE_CXX_FLAGS and CMAKE_C_FLAGS so that
 # 'config-ix' use them during feature checks. It also adds them to both
 # 'LIBCXX_COMPILE_FLAGS' and 'LIBCXX_LINK_FLAGS'
 add_target_flags_if(LIBCXX_BUILD_32_BITS "-m32")
 
 if(LIBCXX_TARGET_TRIPLE)
-  add_target_flags("--target=${LIBCXX_TARGET_TRIPLE}")
+  add_target_flags_if_supported("--target=${LIBCXX_TARGET_TRIPLE}")
 elseif(CMAKE_CXX_COMPILER_TARGET)
   set(LIBCXX_TARGET_TRIPLE "${CMAKE_CXX_COMPILER_TARGET}")
 endif()
 if(LIBCXX_SYSROOT)
-  add_target_flags("--sysroot=${LIBCXX_SYSROOT}")
+  add_target_flags_if_supported("--sysroot=${LIBCXX_SYSROOT}")
 elseif(CMAKE_SYSROOT)
   set(LIBCXX_SYSROOT "${CMAKE_SYSROOT}")
 endif()
 if(LIBCXX_GCC_TOOLCHAIN)
-  add_target_flags("--gcc-toolchain=${LIBCXX_GCC_TOOLCHAIN}")
+  add_target_flags_if_supported("--gcc-toolchain=${LIBCXX_GCC_TOOLCHAIN}")
 elseif(CMAKE_CXX_COMPILER_EXTERNAL_TOOLCHAIN)
   set(LIBCXX_GCC_TOOLCHAIN "${CMAKE_CXX_COMPILER_EXTERNAL_TOOLCHAIN}")
 endif()
 
 if(LIBCXX_TARGET_TRIPLE)
   set(TARGET_TRIPLE "${LIBCXX_TARGET_TRIPLE}")
 endif()
 

libcxx/cmake/Modules/HandleLibcxxFlags.cmake

 # all of 'CMAKE_CXX_FLAGS', 'CMAKE_C_FLAGS', 'LIBCXX_COMPILE_FLAGS'
 # and 'LIBCXX_LINK_FLAGS'.
 macro(add_target_flags_if condition)
   if (${condition})
     add_target_flags(${ARGN})
   endif()
 endmacro()
 
+# Add all the flags supported by the compiler to all of
+# 'CMAKE_CXX_FLAGS', 'CMAKE_C_FLAGS', 'LIBCXX_COMPILE_FLAGS'
+# and 'LIBCXX_LINK_FLAGS'.
+macro(add_target_flags_if_supported)
+  foreach(flag ${ARGN})
+    mangle_name("${flag}" flagname)
+    check_cxx_compiler_flag("${flag}" "LIBCXX_SUPPORTS_${flagname}_FLAG")
+    add_target_flags_if(LIBCXX_SUPPORTS_${flagname}_FLAG ${flag})
+  endforeach()
+endmacro()
+
 # Add a specified list of flags to both 'LIBCXX_COMPILE_FLAGS' and
 # 'LIBCXX_LINK_FLAGS'.
 macro(add_flags)
   foreach(value ${ARGN})
     list(APPEND LIBCXX_COMPILE_FLAGS ${value})
     list(APPEND LIBCXX_LINK_FLAGS ${value})
   endforeach()
 endmacro()

libcxxabi/CMakeLists.txt

 #===============================================================================
 # Setup Compiler Flags
 #===============================================================================
 
 # Configure target flags
 add_target_flags_if(LIBCXXABI_BUILD_32_BITS "-m32")
 
 if(LIBCXXABI_TARGET_TRIPLE)
-  add_target_flags("--target=${LIBCXXABI_TARGET_TRIPLE}")
+  add_target_flags_if_supported("--target=${LIBCXXABI_TARGET_TRIPLE}")
 elseif(CMAKE_CXX_COMPILER_TARGET)
   set(LIBCXXABI_TARGET_TRIPLE "${CMAKE_CXX_COMPILER_TARGET}")
 endif()
 if(LIBCXX_GCC_TOOLCHAIN)
-  add_target_flags("--gcc-toolchain=${LIBCXXABI_GCC_TOOLCHAIN}")
+  add_target_flags_if_supported("--gcc-toolchain=${LIBCXXABI_GCC_TOOLCHAIN}")
 elseif(CMAKE_CXX_COMPILER_EXTERNAL_TOOLCHAIN)
   set(LIBCXXABI_GCC_TOOLCHAIN "${CMAKE_CXX_COMPILER_EXTERNAL_TOOLCHAIN}")
 endif()
 if(LIBCXXABI_SYSROOT)
-  add_target_flags("--sysroot=${LIBCXXABI_SYSROOT}")
+  add_target_flags_if_supported("--sysroot=${LIBCXXABI_SYSROOT}")
 elseif(CMAKE_SYSROOT)
   set(LIBCXXABI_SYSROOT "${CMAKE_SYSROOT}")
 endif()
 
 if (LIBCXXABI_TARGET_TRIPLE)
   set(TARGET_TRIPLE "${LIBCXXABI_TARGET_TRIPLE}")
 endif()
 

libcxxabi/cmake/Modules/HandleLibcxxabiFlags.cmake

 # all of 'CMAKE_CXX_FLAGS', 'CMAKE_C_FLAGS', 'LIBCXXABI_COMPILE_FLAGS'
 # and 'LIBCXXABI_LINK_FLAGS'.
 macro(add_target_flags_if condition)
   if (${condition})
     add_target_flags(${ARGN})
   endif()
 endmacro()
 
+# Add all the flags supported by the compiler to all of
+# 'CMAKE_CXX_FLAGS', 'CMAKE_C_FLAGS', 'LIBCXXABI_COMPILE_FLAGS'
+# and 'LIBCXXABI_LINK_FLAGS'.
+macro(add_target_flags_if_supported)
+  foreach(flag ${ARGN})
+    mangle_name("${flag}" flagname)
+    check_cxx_compiler_flag("${flag}" "LIBCXXABI_SUPPORTS_${flagname}_FLAG")
+    add_target_flags_if(LIBCXXABI_SUPPORTS_${flagname}_FLAG ${flag})
+  endforeach()
+endmacro()
+
 # Add a specified list of flags to both 'LIBCXXABI_COMPILE_FLAGS' and
 # 'LIBCXXABI_LINK_FLAGS'.
 macro(add_flags)
   foreach(value ${ARGN})
     list(APPEND LIBCXXABI_COMPILE_FLAGS ${value})
     list(APPEND LIBCXXABI_LINK_FLAGS ${value})
   endforeach()
 endmacro()

libunwind/CMakeLists.txt

 #===============================================================================
 # Setup Compiler Flags
 #===============================================================================
 
 # Get required flags.
 add_target_flags_if(LIBUNWIND_BUILD_32_BITS "-m32")
 
 if(LIBUNWIND_TARGET_TRIPLE)
-  add_target_flags("--target=${LIBUNWIND_TARGET_TRIPLE}")
+  add_target_flags_if_supported("--target=${LIBUNWIND_TARGET_TRIPLE}")
 elseif(CMAKE_CXX_COMPILER_TARGET)
   set(LIBUNWIND_TARGET_TRIPLE "${CMAKE_CXX_COMPILER_TARGET}")
 endif()
 if(LIBUNWIND_GCC_TOOLCHAIN)
-  add_target_flags("--gcc-toolchain=${LIBUNWIND_GCC_TOOLCHAIN}")
+  add_target_flags_if_supported("--gcc-toolchain=${LIBUNWIND_GCC_TOOLCHAIN}")
 elseif(CMAKE_CXX_COMPILER_EXTERNAL_TOOLCHAIN)
   set(LIBUNWIND_GCC_TOOLCHAIN "${CMAKE_CXX_COMPILER_EXTERNAL_TOOLCHAIN}")
 endif()
 if(LIBUNWIND_SYSROOT)
-  add_target_flags("--sysroot=${LIBUNWIND_SYSROOT}")
+  add_target_flags_if_supported("--sysroot=${LIBUNWIND_SYSROOT}")
 elseif(CMAKE_SYSROOT)
   set(LIBUNWIND_SYSROOT "${CMAKE_SYSROOT}")
 endif()
 
 if (LIBUNWIND_TARGET_TRIPLE)
   set(TARGET_TRIPLE "${LIBUNWIND_TARGET_TRIPLE}")
 endif()
 

libunwind/cmake/Modules/HandleLibunwindFlags.cmake

 # all of 'CMAKE_CXX_FLAGS', 'CMAKE_C_FLAGS', 'LIBUNWIND_COMPILE_FLAGS'
 # and 'LIBUNWIND_LINK_FLAGS'.
 macro(add_target_flags_if condition)
   if (${condition})
     add_target_flags(${ARGN})
   endif()
 endmacro()
 
+# Add all the flags supported by the compiler to all of
+# 'CMAKE_CXX_FLAGS', 'CMAKE_C_FLAGS', 'LIBUNWIND_COMPILE_FLAGS'
+# and 'LIBUNWIND_LINK_FLAGS'.
+macro(add_target_flags_if_supported)
+  foreach(flag ${ARGN})
+    mangle_name("${flag}" flagname)
+    check_cxx_compiler_flag("${flag}" "LIBUNWIND_SUPPORTS_${flagname}_FLAG")
+    add_target_flags_if(LIBUNWIND_SUPPORTS_${flagname}_FLAG ${flag})
+  endforeach()
+endmacro()
+
 # Add a specified list of flags to both 'LIBUNWIND_COMPILE_FLAGS' and
 # 'LIBUNWIND_LINK_FLAGS'.
 macro(add_flags)
   foreach(value ${ARGN})
     list(APPEND LIBUNWIND_COMPILE_FLAGS ${value})
     list(APPEND LIBUNWIND_LINK_FLAGS ${value})
   endforeach()
 endmacro()